Hazardous and solid wastes containing heavy metal contaminants are a major environmental issue. Many of these wastes are in landfills that are potentially leaching heavy metals into water supplies. Attempts have been made to protect the environment from contamination, but many of these attempts are expensive or do not adequately prevent the heavy metal contaminants from leaching into the environment.
Coal ash, including coal combustion products (CCPs) and coal combustion residue (CCRs), and electric arc furnace dust (EAFD) present major waste problems in the US and other industrialized nations. Both have been shown to contain heavy metals (lead, mercury, iron, copper, manganese, cadmium, arsenic, nickel, aluminum, silver, and beryllium) and light metals and halides (sodium, lithium, sulfur, chloride, etc.) that are or can be hazardous if not handled properly.
Coal Ash is solid wastes generated during the burning of coal in power plants. Coal Ash includes fly ash, bottom ash, boiler slag, grit, flue gas desulfurization material and other wastes, such as fluidized bed combustion ash, cenospheres, and scrubber residues. Of the Coal Ash produced from burning coal for power generation, over 80% ends up as fly ash with much of the remainder being bottom ash. Coal Ash contains organic carbon materials (through loss on ignition) and metal contaminants, such as arsenic, boron, cadmium, chromium, copper, lead, selenium, zinc and the like.
EAFD is a byproduct of the melting of metals in an electric arc furnace to make carbon steel. EAFD forms as a result of the molten metal being oxidized and cooled. Over 2% of the scrap feed melted to produce carbon steel ends up as EAFD. EAFD contains free and composite oxides. The composite oxides include magnesium, iron, nickel, manganese, zinc, copper, chromium, cadmium, lead and/or chlorides. EAFD is classified by the U.S. Environmental Protection Agency as a K061 hazardous waste because it fails that agency's toxicity characteristic leaching procedure test for lead, cadmium and chromium.
Currently, Coal Ash are not classified as hazardous waste. Most Coal Ash is disposed of in landfills or ash ponds, but some are recycled. Fly ash, for example, is used extensively in concrete and soil stabilization. The chemical makeup of Coal Ash depends on the compounds in the burned coal, but most contain amounts of silicon dioxide (SiO2) and calcium oxide (CaO). Fly ash is classified as Class F or Class C depending upon the amount of calcium, silica, alumina, and iron content in the ash. Class F fly ash requires an activator and is used in Portland cement. Class C fly ash is self-cementing and does not require a chemical reagent or activator.
Disposing of EAFD is more complicated due to the hazardous waste classification. Currently, some EAFD is sent to a landfill, but most is treated by fixing the metal in a solid or processing at high temperatures or another electric or hydrometallurgical process to separate the metals out of the dust. However, current methods of processing EAFD are expensive and may be ineffective or hazardous themselves.
Abrasives are compounds used to shape, smooth and polish other materials. Abrasives are also used to remove a substance (such as paint) from a surface. Common uses for abrasives include grinding, polishing, buffing, honing, cutting, drilling, sharpening, lapping, and sanding. Abrasives are compounds that are harder than the materials they shape, smooth, polish or remove.
U.S. Pat. No. 6,057,257 to Morano, et al., teaches an abrasive composed of EAFD, glass, and silica sand. U.S. Pat. No. 5,964,911 to Morano, et al., teaches a process for making an abrasive composition by melting a mixture of EAFD, glass, and silica sand. These patents teach adding glass compositions, crushed glass, or cullet. The disclosures of each of these patents is hereby incorporated by reference into this specification.
There is a strong need for a safe, economical way to prevent the heavy metal contaminants in Coal Ash and EAFD from leaching into the environment, while at the same time creating useful commercial products. There is a need for abrasives and other glass ceramic products that perform well but are reasonably priced. No products currently on the market derive from a combination of Coal Ash and EAFD. It is an object of this invention to recycle Coal Ash and EAFD to create a feedstock for the manufacture of useful commercial products, such as economical abrasives and other glass ceramic products. It is an object of this invention to create a crude feedstock made from Coal Ash and EAFD which is environmentally safe. It is another object of this invention to produce a effective abrasives and other glass ceramic compositions with controlled hardness and fracture toughness properties that are inexpensive to produce and that outperform other products currently in the market. It is an object of the invention to use Coal Ash that was previously destined for landfills, as a readily available, high volume, cheap source of iron, alumina and silica to produce abrasives and other glass-ceramic products.